Derivatization of biologically active peptides by conjugation with fluorophores or radionuclide-bearing moieties is an effective and commonly used approach to prepare molecular tools and diagnostic agents. Whereas lysine, cysteine, and N-terminal amino acids have been mostly used for peptide conjugation, we describe a new, widely applicable approach to peptide conjugation based on the nonclassical bioisosteric replacement of the guanidine group in arginine by a functionalized carbamoylguanidine moiety. Four arginine-containing peptide receptor ligands (angiotensin II, neurotensin(8-13), an analogue of the C-terminal pentapeptide of neuropeptide Y, and a neuropeptide FF analogue) were subject of this proof-of-concept study. The N(ω)-carbamoylated arginines, bearing spacers with a terminal amino group, were incorporated into the peptides by standard Fmoc solid phase peptide synthesis. The synthesized chemically stable peptide derivatives showed high receptor affinities with Ki values in the low nanomolar range, even when bulky fluorophores had been attached. Two new tritiated tracers for angiotensin and neurotensin receptors are described.

A series of new piperidinomethylphenoxypropylamine-type histamine H2 receptor (H2 R) antagonists with different substituted "urea equivalents" was synthesized and characterized in functional in vitro assays. Based on these data as selection criteria, radiosynthesis of N-[6-(3,4-dioxo-2-{3-[3-(piperidin-1-ylmethyl)phenoxy]propylamino}cyclobut-1-enylamino)hexyl]-(2,3-(3) H2 )propionic amide ([(3) H]UR-DE257) was performed. The radioligand (specific activity: 63 Ci mmol(-1) ) had high affinity for human, rat, and guinea pig H2 R (hH2 R, Sf9 cells: Kd , saturation binding: 31 nM, kinetic studies: 20 nM). UR-DE257 revealed high H2 R selectivity on membranes of Sf9 cells, expressing the respective hHx R subtype (Ki values: hH1 R: >10000 nM, hH2 R: 28 nM, hH3 R: 3800 nM, hH4 R: >10000 nM). In spite of insurmountable antagonism, probably due to rebinding of [(3) H]UR-DE257 to the H2 R (extended residence time), the title compound proved to be a valuable pharmacological tool for the determination of H2 R affinities in competition binding assays.

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Fluorescence labeled ligands have been gaining importance as molecular tools, enabling receptor–ligand-binding studies by various fluorescence-based techniques. Aiming at red-emitting fluorescent ligands for the hH2R, a series of squaramides labeled with pyridinium or cyanine fluorophores (19–27) was synthesized and characterized. The highest hH2R affinities in radioligand competition binding assays were obtained in the case of pyridinium labeled antagonists 19–21 (pK i: 7.71–7.76) and cyanine labeled antagonists 23 and 25 (pK i: 7.67, 7.11). These fluorescent ligands proved to be useful tools for binding studies (saturation and competition binding as well as kinetic experiments), using confocal microscopy, flow cytometry, and high content imaging. Saturation binding experiments revealed pK d values comparable to the pK i values. The fluorescent probes 21, 23, and 25 could be used to localize H2 receptors in HEK cells and to determine the binding affinities of unlabeled compounds.

The back cover picture shows [3H]UR‐DE257, a tritium‐labeled selective histamine H2 receptor (H2R) antagonist identified from a series of piperidinomethylphenoxypropylamines. [3H]UR‐DE257 exhibits high specific binding and comparable Kd values at the human, rat, and guinea pig H2R. Despite insurmountable antagonism in functional studies and slow dissociation, it is fully displaceable in competition binding studies. Thus, [3H]UR‐DE257 is a valuable tool for the study of H2R. For more details, see the Full Paper by Armin Buschauer et al. on